= State of matter
{wiki}
A state of matter is a broad category of material behavior. A <solid> resists a sustained change of shape, a <liquid> flows while remaining difficult to compress, and a <gas> expands to fill its container. These distinctions are useful first descriptions, but they do not enumerate all <thermodynamic phases>. <Plasma> adds collective behavior of charged particles, while <liquid crystals> show that flow and structural order need not disappear together.
Distinguish three questions: what broad state of matter does the material resemble, which <thermodynamic phase> is present, and what are its <temperature>, <pressure> and other state variables? Two samples at different <temperatures> can be in the same <liquid> <thermodynamic phase>. Conversely, <crystal polymorphism> gives different <solid> <thermodynamic phases> of the same substance, distinguished by their <crystal structures>. A <phase transition> between them need not change the broad state-of-matter category.
For an imagined substance with crystalline forms A and B, melting A produces a <solid>–<liquid> <phase transition>, while converting A to B produces a <solid>–<solid> <phase transition>. A sample containing A and B has two <thermodynamic phases> even though all its material is <solid>. Cutting one uniform crystal into two pieces does not create a second <thermodynamic phase>: count distinct equilibrium forms, rather than pieces or grains. This distinction matters when applying the <phase rule>.
A <phase diagram> locates stable <thermodynamic phases> using <temperature> and <pressure>. Its <liquid>–<gas> <phase coexistence curve> ends at a <liquid-gas critical point>, around which a path connects <liquid> and <gas> without a <first-order phase transition>. Broad categories can therefore remain useful without a sharp boundary.<Statistical Physics by David Tong>[§5.1.2, printed pp. 140–141, Figures 40–41: pressure–temperature coexistence and disappearance of the liquid–gas distinction]{ref}
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